Sialic acids (Sias) presented at the distal ends of vertebrate glycan chains mediate many biological roles, including binding by intrinsic Sia-recognizing receptors called Siglecs (Sia-recognizing Ig-like lectins). We have hypothesized that the CD33-related subset of Siglecs (CD33rSiglecs) in primates and rodents recognize host Sias as self, thereby dampening the reactivity of immune cells. We also hypothesize that Sia-expressing bacterial pathogens take advantage of this mechanism to down-regulate innate immune reactivity against them. We earlier discovered a human-specific evolutionary loss of the common mammalian Sia N-glycolylneuraminic acid (NeuSGc). This would have resulted in loss of optimal CD33rSiglec ligands. A variety of human-specific genetic changes and adjustments in these lectins apparently then ensued, leaving the human immune system in an altered state relative to that of our great ape evolutionary relatives. The biological and pathological consequences of these differences are being studied by comparing humans and great apes, but many practical, ethical and fiscal issues limit this approach. We therefore propose to use transgenic mice to model and compare human and chimpanzee sialic acid and CD33rSiglec biology, elucidating functional consequences resulting from genetic changes during human evolution. The overall hypothesis being tested is that the human propensity to develop inflammatory diseases involving innate and adaptive immune cells, as well as infections by Sia-expressing bacteria are related to human-specific evolutionary changes in certain CD33rSiglecs. These include humanspecific changes in Sia-binding properties of Siglec-9 on neutrophils and monocytes; in the binding properties, expression and function of Siglec-11 and -12 on macrophages; of Siglec-11 on human brain microglia, and the selective down-regulation of Siglec-5 on human T cells. We will use a variety of genetically modified mice to mimic the ancestral human condition of constitutive CD33rSiglec unmasking in myelomonocytic cells; the current functional states of human and chimpanzee Siglec-9; human-specific changes in Siglec-11 and -12 on macrophages; human-specific Siglec-11 expression in microglia; and, the ancestral great ape state of Siglec-5 expression on T lymphocytes. These studies will be done in wild-type mice, and in strains deficient in the relevant murine CD33rSiglecs. Appropriate studies of innate and adaptive immune responses as well as challenges with Sia-expressing bacterial pathogens will test the original hypotheses. These studies involve collaborations with other program members and utilize all Core services in the program. Importantly, even if some of the taken approaches fail to accurately mirror human evolution, the results will illuminate various general underlying principles regarding the biology of CD33rSiglecs on innate and adaptive immune blood cells.